Optical coherence tomography (“OCT”) and techniques and arrangement which are similar to derivative thereof have become promising techniques for obtaining high-resolution cross-sectional images of human tissue in vivo. One manner for conducting OCT may be based on time domain OCT scanning (“TD-OCT”). In this exemplary method, the length of the reference arm in an interferometer can be rapidly scanned over a distance corresponding to the imaging depth range.
FIG. 1 shows an exemplary system which can utilize the TD-OCT techniques. Thus, upon the rapid scanning of a reference arm 25 of an interferometer, an interference pattern may be produced, e.g., only when the path length of the reference arm 25 matches the path-length to a given scatterer in a sample arm 45 to within the temporal coherence length of the source 10 light. When utilizing TD-OCT techniques, a broad bandwidth light 10 may be input into the interferometer, and split into the reference arm 25 and the sample arm 45. The optical path length of the reference arm 25 is scanned by translating a reference arm mirror 20. The light returned from the reference and sample arms 25, 45 can combine at a splitter 30. Interference fringes may be detected when the sample arm path length matches the reference arm path length to within the coherence length of the light. A detection of the fringe patterns allows one axial scan (e.g., an A-line) to be generated that maps tissue reflectivity to a given axial or depth location. An image may be generated by repeating this process at successive transverse locations on a sample 60.
Alternative techniques that conduct ranging in the frequency domain, such as spectral-domain OCT (“SD-OCT”) and optical frequency domain imaging (“OFDI”) do not substantially move the reference arm, but instead detect the spectral interference pattern. The optical imaging engine, system and method which can perform optical frequency domain imaging (“OFDI”) and the optical imaging engine, system and method which can perform spectral-domain optical coherence tomography are described in U.S. Provisional Patent Appn. No. 60/514,769 filed Oct. 27, 2003 and International Patent Application No. PCT/US03/02349 filed on Jan. 24, 2003, respectively, the entire disclosure of which is incorporated herein by reference. Fourier transformation of such spectral interference pattern can result in an axial reflectivity profile. SD-OCT and OFDI techniques may be becoming preferred modes of use for OCT imaging as they likely have an inherent sensitivity advantage over the TD-OCT techniques. The frequency domain methods furthermore may not require a rapidly scanning optical delay line to achieve imaging at high frame rates. As a result, the exemplary SD-OCT and OFDI imaging techniques can be conducted at higher speeds than their time-domain analogs. A common feature of the time- and frequency-domain OCT systems is that they can be implemented using fiber optics. Another common feature of both the time- and frequency-domain OCT imaging systems is that the sample and reference arms generally travel along distinct paths following the beam splitting mechanism.
A significant number of the OCT systems require scanning of the sample arm beam while obtaining axial reflectivity profiles to construct a two-dimensional cross-sectional image of the sample. For ophthalmology and dermatologic applications, the sample arm scanner may consist of a hand-held galvanometric or resonant scanner. For access to internal organs, fiber optic catheters and endoscopes have been developed that reside in the sample arm of the OCT interferometer.
FIG. 2 shows an exemplary configuration of mechanical and optical elements that are used in a number of OCT catheters/endoscopes. Such catheter configurations may contain an inner core 120, which consists contains a fiber optic element 115 such as an optical fiber that is coupled to the OCT system by an optical coupler 110 at the proximal end and distal optics 140 that focus and redirects the light at the distal end 150 onto the sample 160. The inner core 120 can rotate or translate to provide one-dimensional scanning. The inner core is typically enclosed in a transparent sheath 130 (FIG. 2).
Certain Problems Associated with Conventional Fiber Optic OCT Probes
Path length mismatches. The use of optical fiber based catheters in OCT is problematic due to artifacts that may occur when the catheter is bent or twisted while scanning the OCT beam on the sample. For example, even for small bends of the catheter, the image tends to drift off of the screen. This effect occurs because twisting or bending the optical fiber within the catheter causes a path length mismatch between the sample and reference arm. As a result, the operator of the OCT system continually adjusts the reference arm path length to keep the image on screen.
Dispersion mismatches. Another significant artifact may be a loss of signal strength and resolution due to dispersion imbalances between the sample arm and reference arm. Dispersion is a degree of difference in velocity for each wavelength of light as it passes through a material such as the optical fiber, focusing lens and prism of the catheter. This problem becomes even more significant as higher resolution imaging is attempted, as the reference and sample arm dispersion needs to be more stringently matched to maintain high-resolution imaging.
Polarization mismatches. Another artifact that may occur while bending or twisting the fiber in the body is change of polarization and polarization mode dispersion within the sample arm. Polarization mode dispersion is the degree of different light velocities experienced by the two orthogonal polarization states present in the fiber. Since the reference arm does not undergo similar changes, loss of resolution and signal strength may occur when polarization is dynamically altered within the sample arm catheter alone.